Microelectronic programmable device and methods of forming and programming the same

ABSTRACT

A microelectronic programmable structure and methods of forming and programming the structure are disclosed. The programmable structure generally include an ion conductor and a plurality of electrodes. Electrical properties of the structure may be altered by applying a bias across the electrodes, and thus information may be stored using the structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/951,882,entitled MICROELECTRONIC PROGRAMMABLE DEVICE AND METHODS OF FORMING ANDPROGRAMMING THE SAME, filed Sep. 10, 2001, now U.S. Pat. No. 6,635,914,which is a Continuation in Part of U.S. patent application Ser. No.09/502,915, entitled PROGRAMMABLE MICROELECTRONIC DEVICES AND METHODS OFFORMING AND PROGRAMMING SAME, filed Feb. 11, 2000, now U.S. Pat. No.6,487,106, which is a Continuation in Part of U.S. patent applicationSer. No. 09/555,612, entitled PROGRAMMABLE SUBSURFACE AGGREGATINGMETALLIZATION CELL STRUCTURE AND METHOD OF MAKING SAME, filed Dec. 4,1998, now U.S. Pat. No. 6,418,049; and which claims the benefit of; U.S.patent application Ser. No. 60/231,343, entitled COMMON ELECTRODECONFIGURATIONS OF THE PROGRAMMABLE METALLIZATION CELL, filed Sep. 8,2000; U.S. patent application Ser. No. 60/231,345, entitled GLASSCOMPOSITION SUITABLE FOR PROGRAMMABLE METALLIZATION CELLS AND METHOD OFFORMING THE SAME, filed Sep. 8, 2000; U.S. patent application Ser. No.60/231,350, entitled ULTRA LOW ENERGY PROGRAMMABLE METALLIZATION CELLDEVICES AND METHODS OF FORMING THE SAME, filed Sep. 8, 2000; U.S. patentapplication Ser. No. 60/231,427, entitled ELECTRODES FOR THEPROGRAMMABLE METALLIZATION CELL, filed Sep. 8, 2000; U.S. patentapplication Ser. No. 60/231,346, entitled SOLID SOLUTION FOR THEPROGRAMMABLE METALLIZATION CELL AND METHOD OF FORMING THE SAME, filedSep. 8, 2000; U.S. patent application Ser. No. 60/231,432, entitledPROGRAMMABLE METALLIZATION CELL WITH FLOATING ELECTRODE AND METHOD OFPROGRAMMING AND FORMING THE SAME, filed Sep. 8, 2000; U.S. patentapplication Ser. No. 60/282,045, entitled OPTIMIZED ELECTRODES FOR THEPROGRAMMABLE METALLIZATION CELL, filed Apr. 6, 2001; U.S. patentapplication Ser. No. 60/283,591, entitled OPTIMIZED GLASS COMPOSITIONFOR THE PROGRAMMABLE METALLIZATION CELL, filed Apr. 13, 2001; and U.S.patent application Ser. No. 60/291,886, entitled ELECTRODES FOR THEPROGRAMMABLE METALLIZATION CELL, filed May 18, 2001.

FIELD OF THE INVENTION

The present invention generally relates to microelectronic devices. Moreparticularly, the invention relates to programmable microelectronicstructures suitable for use in integrated circuits.

BACKGROUND OF THE INVENTION

Memory devices are often used in electronic systems and computers tostore information in the form of binary data. These memory devices maybe characterized into various types, each type having associated with itvarious advantages and disadvantages.

For example, random access memory (“RAM”) which may be found in personalcomputers is typically volatile semiconductor memory; in other words,the stored data is lost if the power source is disconnected or removed.Dynamic RAM (“DRAM”) is particularly volatile in that it must be“refreshed” (i.e., recharged) every few microseconds in order tomaintain the stored data. Static RAM (“SRAM”) will hold the data afterone writing so long as the power source is maintained; once the powersource is disconnected, however, the data is lost. Thus, in thesevolatile memory configurations, information is only retained so long asthe power to the system is not turned off In general, these RAM devicescan take up significant chip area and therefore may be expensive tomanufacture and consume relatively large amounts of energy for datastorage. Accordingly, improved memory devices suitable for use inpersonal computers and the like are desirable.

Other storage devices such as magnetic storage devices (e.g., floppydisks, hard disks and magnetic tape) as well as other systems, such asoptical disks, CD-RW and DVD-RW are non-volatile, have extremely highcapacity, and can be rewritten many times. Unfortunately, these memorydevices are physically large, are shock/vibration-sensitive, requireexpensive mechanical drives, and may consume relatively large amounts ofpower. These negative aspects make such memory devices non-ideal for lowpower portable applications such as lap-top and palm-top computers,personal digital assistants (“PDAs”), and the like.

Due, at least in part, to a rapidly growing numbers of compact,low-power portable computer systems in which stored information changesregularly, low energy read/write semiconductor memories have becomeincreasingly desirable and widespread. Furthermore, because theseportable systems often require data storage when the power is turnedoff, non-volatile storage device are desired for use in such systems.

One type of programmable semiconductor non-volatile memory devicesuitable for use in such systems is a programmable read-only memory(“PROM”) device. One type of PROM, a write-once read-many (“WORM”)device, uses an array of fusible links. Once programmed, the WORM devicecannot be reprogrammed.

Other forms of PROM devices include erasable PROM (“EPROM”) andelectrically erasable PROM (EEPROM) devices, which are alterable afteran initial programming. EPROM devices generally require an erase stepinvolving exposure to ultra violet light prior to programming thedevice. Thus, such devices are generally not well suited for use inportable electronic devices. EEPROM devices are generally easier toprogram, but suffer from other deficiencies. In particular, EEPROMdevices are relatively complex, are relatively difficult to manufacture,and are relatively large. Furthermore, a circuit including EEPROMdevices must withstand the high voltages necessary to program thedevice. Consequently, EEPROM cost per bit of memory capacity isextremely high compared with other means of data storage. Anotherdisadvantage of EEPROM devices is that, although they can retain datawithout having the power source connected, they require relatively largeamounts of power to program. This power drain can be considerable in acompact portable system powered by a battery.

In view of the various problems associated with conventional datastorage devices described above, a relatively non-volatile, programmabledevice which is relatively simple and inexpensive to produce is desired.Furthermore, this memory technology should meet the requirements of thenew generation of portable computer devices by operating at a relativelylow voltage while providing high storage density and a low manufacturingcost.

SUMMARY OF THE INVENTION

The present invention provides improved microelectronic devices for usein integrated circuits. More particularly, the invention providesrelatively non-volatile, programmable devices suitable for memory andother integrated circuits.

The ways in which the present invention addresses various drawbacks ofnow-known programmable devices are discussed in greater detail below.However, in general, the present invention provides a programmabledevice that is relatively easy and inexpensive to manufacture, and whichis relatively easy to program.

In accordance with one exemplary embodiment of the present invention, aprogrammable structure includes an ion conductor and at least twoelectrodes. The structure is configured such that when a bias is appliedacross two electrodes, one or more electrical properties of thestructure change. In accordance with one aspect of this embodiment, aresistance across the structure changes when a bias is applied acrossthe electrodes. In accordance with other aspects of this embodiment, acapacitance or other electrical property of the structure changes uponapplication of a bias across the electrodes. One or more of theseelectrical changes may suitably be detected. Thus, stored informationmay be retrieved from a circuit including the structure.

In accordance with another exemplary embodiment of the invention, aprogrammable structure includes an ion conductor, at least twoelectrodes, and a barrier interposed between at least a portion of oneof the electrodes and the ion conductor. In accordance with one aspectof this embodiment the barrier material includes a material configuredto reduce diffusion of ions between the ion conductor and at least oneelectrode. The diffusion barrier may also serve to prevent undesiredelectrodeposit growth within a portion of the structure. In accordancewith another aspect, the barrier material includes an insulatingmaterial. Inclusion of an insulating material increases the voltagerequired to reduce the resistance of the device. In accordance with yetanother aspect of this embodiment, the barrier includes material thatconducts ions, but which is relatively resistant to the conduction ofelectrons. Use of such material may reduce undesired plating at anelectrode and increase the thermal stability of the device.

In accordance with another exemplary embodiment of the invention, aprogrammable microelectronic structure is formed on a surface of asubstrate by forming a first electrode on the substrate, depositing alayer of ion conductor material over the first electrode, and depositingconductive material onto the ion conductor material. In accordance withone aspect of this embodiment, a solid solution including the ionconductor and excess conductive material is formed by dissolving (e.g.,via thermal and/or photodissolution) a portion of the conductivematerial in the ion conductor. In accordance with a further aspect, onlya portion of the conductive material is dissolved, such that a portionof the conductive material remains on a surface of the ion conductor toform an electrode on a surface of the ion conductor material.

In accordance with another embodiment of the present invention, at leasta portion of a programmable structure is formed within a through-hole orvia in an insulating material. In accordance with one aspect of thisembodiment, a first electrode feature is formed on a surface of asubstrate, insulating material is deposited onto a surface of theelectrode feature, a via is formed within the insulating material, and aportion of the programmable structure is formed within the via. Afterthe via is formed within the insulating material, a portion of thestructure within the via is formed by depositing an ion conductivematerial onto the conductive material, depositing a second electrodematerial onto the ion conductive material, and, if desired, removing anyexcess electrode, ion conductor, and/or insulating material. Inaccordance with another aspect of this embodiment, only the ionconductor is formed within the via. In this case, a first electrode isformed below the insulating material an in contact with the ionconductor and the second electrode is formed above the insulatingmaterial and in contact with the ion conductor. The configuration of thevia may be changed to alter (e.g., reduce) a contact area between one ormore of the electrodes and the ion conductor. Reducing thecross-sectional area of the interface between the ion conductor and theelectrode increases the efficiency of the device (change in electricalproperty per amount of power supplied to the device). In accordance withanother aspect of this embodiment, the via may extend through the lowerelectrode to reduce the interface area between the electrode and the ionconductor. In accordance with yet another aspect of this embodiment, aportion of the ion conductor may be removed from the via or the ionconductor material may be directionally deposited into only a portion ofthe via to further reduce an interface between an electrode and the ionconductor.

In accordance with another embodiment of the invention, a programmabledevice may be formed on a surface of a substrate.

In accordance with a further exemplary embodiment of the invention,multiple bits of information are stored in a single programmablestructure. In accordance with one aspect of this embodiment, aprogrammable structure includes a floating electrode interposed betweentwo additional electrodes.

In accordance with yet another embodiment of the invention, multipleprogrammable devices are coupled together using a common electrode(e.g., a common anode or a common cathode).

In accordance with yet another embodiment of the invention, multipleprogrammable devices share a common electrode.

In accordance with yet a further exemplary embodiment of the presentinvention, a capacitance of a programmable structure is altered bycausing ions within an ion conductor of the structure to migrate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims, considered inconnection with the figures, wherein like reference numbers refer tosimilar elements throughout the figures, and:

FIG. 1 is a cross-sectional illustration of a programmable structureformed on a surface of a substrate in accordance with the presentinvention;

FIG. 2 is a cross-sectional illustration of a programmable structure inaccordance with an alternative embodiment of the present invention;

FIG. 3 is a current-voltage diagram illustrating current and voltagecharacteristics of the device illustrated in FIG. 2 in an “on” and “off”state;

FIG. 4 is a cross-sectional illustration of a programmable structure inaccordance with yet another embodiment of the present invention;

FIG. 5 is a schematic illustration of a portion of a memory device inaccordance with an exemplary embodiment of the present invention;

FIG. 6 is a schematic illustration of a portion of a memory device inaccordance with an alternative embodiment of the present invention;

FIGS. 7 and 8 are a cross-sectional illustrations of a programmablestructure having an ion conductor/electrode contact interface formedabout a perimeter of the ion conductor in accordance with anotherembodiment of the present invention;

FIGS. 9 and 10 are a cross-sectional illustrations of a programmablestructure having an ion conductor/electrode contact interface formedabout a perimeter of the ion conductor in accordance with yet anotherembodiment of the present invention;

FIGS. 11 and 12 illustrate a programmable device having a horizontalconfiguration in accordance with the present invention;

FIGS. 13–19 illustrate programmable device structures with reducedelectrode/ion conductor interface surface area in accordance with thepresent invention;

FIG. 20 illustrates a programmable device with a tapered ion conductorin accordance with the present invention;

FIGS. 21–24 illustrate a programmable device including a floatingelectrode in accordance with the present invention; and

FIGS. 25–29 illustrate common electrode programmable device structuresin accordance with the present invention.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention generally relates to microelectronic devices. Moreparticularly, the invention relates to programmable structures ordevices suitable for various integrated circuit applications.

FIGS. 1 and 2 illustrate programmable microelectronic structures 100 and200 formed on a surface of a substrate 110 in accordance with anexemplary embodiment of the present invention. Structures 100 and 200include electrodes 120 and 130, an ion conductor 140, and optionallyinclude buffer or barrier layers 155 and/or 255.

Generally, structures 100 and 200 are configured such that when a biasgreater than a threshold voltage (V_(T)), discussed in more detailbelow, is applied across electrodes 120 and 130, the electricalproperties of structure 100 change. For example, in accordance with oneembodiment of the invention, as a voltage V≧V_(T) is applied acrosselectrodes 120 and 130, conductive ions within ion conductor 140 beginto migrate and form an electrodeposit (e.g., electrodeposit 160) at ornear the more negative of electrodes 120 and 130; such anelectrodeposit, however, is not required to practice the presentinvention. The term “electrodeposit” as used herein means any areawithin the ion conductor that has an increased concentration of reducedmetal or other conductive material compared to the concentration of suchmaterial in the bulk ion conductor material. As the electrodepositforms, the resistance between electrodes 120 and 130 decreases, andother electrical properties may also change. In the absence of anyinsulating barriers, which are discussed in more detail below, thethreshold voltage required to grow the electrodeposit from one electrodetoward the other and thereby significantly reduce the resistance of thedevice is approximately the redox potential of the system, typically afew hundred millivolts. If the same voltage is applied in reverse, theelectrodeposit will dissolve back into the ion conductor and the devicewill return to a high resistance state. In accordance with otherembodiments of the invention, application of an electric field betweenelectrodes 120 and 130 may cause ions dissolved within conductor 140 tomigrate and thus cause a change in the electrical properties of device100, without the formation of an electrodeposit. Structures 100 and 200may be used to store information and thus may be used in memorycircuits. For example, structure 100 or other programmable structures inaccordance with the present invention may suitably be used in memorydevices to replace DRAM, SRAM, PROM, EPROM, or EEPROM devices. Inaddition, programmable structures of the present invention may be usedfor other applications where programming or changing of electricalproperties of a portion of an electrical circuit are desired.

Substrate 110 may include any suitable material. For example, substrate110 may include semiconductive, conductive, semiinsulative, insulativematerial, or any combination of such materials. In accordance with oneembodiment of the invention, substrate 110 includes an insulatingmaterial 112 and a portion 114 including microelectronic devices formedon a semiconductor substrate. Layers 112 and 114 may be separated byadditional layers (not shown) such as, for example, layers typicallyused to form integrated circuits. Because the programmable structurescan be formed over insulating or other materials, the programmablestructures of the present invention are particularly well suited forapplications where substrate (e.g., semiconductor material) space is apremium.

Electrodes 120 and 130 maybe formed of any suitable conductive material.For example, electrodes 120 and 130 may be formed of doped polysiliconmaterial or metal.

In accordance with one exemplary embodiment of the invention, one ofelectrodes 120 and 130 is formed of a material including a metal thatdissolves in ion conductor 140 when a sufficient bias (V≧V_(T)) isapplied across the electrodes (oxidizable electrode) and the otherelectrode is relatively inert and does not dissolve during operation ofthe programmable device (an indifferent electrode). For example,electrode 120 may be an anode during a write process and be comprised ofa material including silver that dissolves in ion conductor 140 andelectrode 130 may be a cathode during the write process and be comprisedof an inert material such as tungsten, nickel, molybdenum, platinum,metal silicides, and the like. Having at least one electrode formed of amaterial including a metal which dissolves in ion conductor 140facilitates maintaining a desired dissolved metal concentration withinion conductor 140, which in turn facilitates rapid and stableelectrodeposit 160 formation within ion conductor 140 or otherelectrical property change during use of structure 100 and/or 200.Furthermore, use of an inert material for the other electrode (cathodeduring a write operation) facilitates electrodissolution of anyelectrodeposit that may have formed and/or return of the programmabledevice to an erased state after application of a sufficient voltage.

During an erase operation, dissolution of any electrodeposit that mayhave formed preferably begins at or near the oxidizableelectrode/electrodeposit interface. Initial dissolution of theelectrodeposit at the oxidizable electrode/electrodeposit interface maybe facilitated by forming structure 100 such that the resistance of theat the oxidizable electrode/electrodeposit interface is greater than theresistance at any other point along the electrodeposit, particularly,the interface between the electrodeposit and the indifferent electrode.

One way to achieve relatively low resistance at the indifferentelectrode is to form the electrode of relatively inert, non-oxidizingmaterial such as platinum. Use of such material reduces formation ofoxides at the interface between ion conductor 140 and the indifferentelectrode as well as the formation of compounds or mixtures of theelectrode material and ion conductor 140 material, which typically havea higher resistance than ion conductor 140 or the electrode material.

Relatively low resistance at the indifferent electrode may also beobtained by forming a barrier layer between the oxidizable electrode(anode during a write operation), wherein the barrier layer is formed ofmaterial having a relatively high resistance. Exemplary high resistancematerials include layers (e.g., layer 155 and/or layer 255) of ionconducting material (e.g., Ag_(x)O, Ag_(x)S, Ag_(x)Se, Ag_(x)Te, wherex≧2, Ag_(y)I, where x≧1, CuI₂, CuO, CuS, CuSe, CuTe, GeO₂, or SiO₂)interposed between ion conductor 140 and a metal layer such as silver.Some of these materials have additional benefits as discussed in moredetail below.

Reliable growth and dissolution of an electrodeposit can also befacilitated by providing a roughened indifferent electrode surface(e.g., a root mean square roughness of greater than about 1 nm) at theelectrode/ion conductor interface. The roughened surface may be formedby manipulating film deposition parameters and/or by etching a portionof one of the electrode of ion conductor surfaces. During a writeoperation, relatively high electrical fields form about the spikes orpeaks of the roughened surface, and thus the electrodeposits are morelikely to form about the spikes or peaks. As a result, more reliable anduniform changes in electrical properties for an applied voltage acrosselectrodes 120 and 130 may be obtained by providing a roughed interfacebetween the indifferent electrode (cathode during a write operation) andion conductor 140.

Oxidizable electrode material may have a tendency to thermally dissolveor diffuse into ion conductor 140, particularly during fabricationand/or operation of structure 100. The thermal diffusion is undesiredbecause it may reduce the resistance of structure 100 and thus reducethe change of an electrical property during use of structure 100.

To reduce undesired diffusion of oxidizable electrode material into ionconductor 140 and in accordance with another embodiment of theinvention, the oxidizable electrode includes a metal intercalated in atransition metal sulfide or selenide material such as A_(x)(MB₂)_(1-x),where A is Ag or Cu, B is S or Se, M is a transition metal such as Ta,V, and Ti, and x ranges from about 0.1 to about 0.7. The intercalatedmaterial mitigates undesired thermal diffusion of the metal (Ag or Cu)into the ion conductor material, while allowing the metal to participatein the electrodeposit growth upon application of a sufficient voltageacross electrodes 120 and 130. For example, when silver in intercalatedinto a TaS₂ film, the TaS₂ film can include up to about 66.8 atomicpercent silver. The A_(x)(MB₂)_(1-x) material is preferably amorphous toprevent to prevent undesired diffusion of the metal though the material.The amorphous material may be formed by, for example, physical vapordeposition of a target material comprising A_(x)(MB₂)_(1-x).

α-AgI is another suitable material for the oxidizable electrode, as wellas the indifferent electrode. Similar to the A_(x)(MB₂)_(1−x) materialdiscussed above, α-AgI can serve as a source of Ag during operation ofstructure 100—e.g., upon application of a sufficient bias, but thesilver in the AgI material does not readily thermally diffuse into ionconductor 140. AgI has a relatively low activation energy for conductionof electricity and does not require doping to achieve relatively highconductivity. When the oxidizable electrode is formed of AgI, depletionof silver in the AgI layer may arise during operation of structure 100,unless excess silver is provided to the electrode. One way to providethe excess silver is to form a silver layer adjacent the AgI layer asdiscussed above when AgI is used as a buffer layer. The AgI layer (e.g.,layer 155 and/or 255) reduces thermal diffusion of Ag into ion conductor140, but does not significantly affect conduction of Ag during operationof structure 100. In addition, use of AgI increases the operationalefficiency of structure 100 because the AgI mitigates non-Faradaicconduction (conduction of electrons that do not participate in theelectrochemical reaction).

Other materials suitable for buffer layers 155 and/or 255 include GeO₂and SiO_(x). Amorphous GeO₂ is relatively porous an will “soak up”silver during operation of device 100, but will retard the thermaldiffusion of silver to ion conductor 140, compared to structures ordevices that do not include a buffer layer. When ion conductor 140includes germanium, GeO₂ may be formed by exposing ion conductor 140 toan oxidizing environment at a temperature of about 300° C. to about 800°C. or by exposing ion conductor 140 to an oxidizing environment in thepresence of radiation having an energy greater than the band gap of theion conductor material. The GeO₂ may also be deposited using physicalvapor deposition (from a GeO₂ target) or chemical vapor deposition (fromGeH₄ and an O₂).

Buffer layers can also be used to increase a “write voltage” by placingthe buffer layer (e.g., GeO₂ or SiO_(x)) between ion conductor 140 andthe indifferent electrode. The buffer material allows metal such assilver to diffuse though the buffer and take part in the electrochemicalreaction.

In accordance with one embodiment of the invention, at least oneelectrode 120 and 130 is formed of material suitable for use as aninterconnect metal. For example, electrode 130 may form part of aninterconnect structure within a semiconductor integrated circuit. Inaccordance with one aspect of this embodiment, electrode 130 is formedof a material that is substantially insoluble in material comprising ionconductor 140. Exemplary materials suitable for both interconnect andelectrode 130 material include metals and compounds such as tungsten,nickel, molybdenum, platinum, metal silicides, and the like.

Layers 155 and/or 255 may also include a material that restrictsmigration of ions between conductor 140 and the electrodes. Inaccordance with exemplary embodiments of the invention, a barrier layerincludes conducting material such as titanium nitride, titaniumtungsten, a combination thereof, or the like. The barrier may beelectrically indifferent, i.e., it allows conduction of electronsthrough structure 100 or 200, but it does not itself contribute ions toconduction through structure 200. An electrically indifferent barriermay reduce undesired dendrite growth during operation of theprogrammable device, and thus may facilitate an “erase” or dissolutionof electrodeposit 160 when a bias is applied which is opposite to thatused to grow the electrodeposit. In addition, use of a conductingbarrier allows for the “indifferent” electrode to be formed ofoxidizable material because the barrier prevents diffusion of theelectrode material to the ion conductor.

Ion conductor 140 is formed of material that conducts ions uponapplication of a sufficient voltage. Suitable materials for ionconductor 140 include glasses and semiconductor materials. In oneexemplary embodiment of the invention, ion conductor 140 is formed ofchalcogenide material.

Ion conductor 140 may also suitably include dissolved conductivematerial. For example, ion conductor 140 may comprise a solid solutionthat includes dissolved metals and/or metal ions. In accordance with oneexemplary embodiment of the invention, conductor 140 includes metaland/or metal ions dissolved in chalcogenide glass. An exemplarychalcogenide glass with dissolved metal in accordance with the presentinvention includes a solid solution of As_(x)S_(1-x)—Ag,Ge_(x)Se_(1-x)—Ag, Ge_(x)S_(1-x)—Ag, As_(x)S_(1-x)—Cu,Ge_(x)Se_(1-x)—Cu, Ge_(x)S_(1-x)—Cu, where x ranges from about 0.1 toabout 0.5 other chalcogenide materials including silver, copper, zinc,combinations of these materials, and the like. In addition, conductor140 may include network modifiers that affects mobility of ions throughconductor 140. For example, materials such as metals (e.g., silver),halogens, halides, or hydrogen may be added to conductor 140 to enhanceion mobility and thus increase erase/write speeds of the structure.

A solid solution suitable for use as ion conductor 140 may be formed ina variety of ways. For example, the solid solution may be formed bydepositing a layer of conductive material such as metal over an ionconductive material such as chalcogenide glass and exposing the metaland glass to thermal and/or photo dissolution processing. In accordancewith one exemplary embodiment of the invention, a solid solution ofAs₂S₃—Ag is formed by depositing As₂S₃ onto a substrate, depositing athin film of Ag onto the As₂S₃, and exposing the films to light havingenergy greater than the optical gap of the As₂S₃,—e.g., light having awavelength of less than about 500 nanometers. If desired, networkmodifiers may be added to conductor 140 during deposition of conductor140 (e.g., the modifier is in the deposited material or present duringconductor 140 material deposition) or after conductor 140 material isdeposited (e.g., by exposing conductor 140 to an atmosphere includingthe network modifier).

In accordance with another embodiment of the invention, a solid solutionmay be formed by depositing one of the constituents onto a substrate oranother material layer and reacting the first constituent with a secondconstituent. For example, germanium (preferably amorphous) may bedeposited onto a portion of a substrate and the germanium may be reactedwith H₂Se to form a Ge—Se glass. Similarly, As can be deposited andreacted with the H₂Se gas, or arsenic or germanium can be deposited andreacted with H₂S gas. Silver or other metal can then be added to theglass as described above.

In accordance with one aspect of this embodiment, a solid solution ionconductor 140 is formed by depositing sufficient metal onto an ionconductor material such that a portion of the metal can be dissolvedwithin the ion conductor material and a portion of the metal remains ona surface of the ion conductor to form an electrode (e.g., electrode120). In accordance with alternative embodiments of the invention, solidsolutions containing dissolved metals may be directly deposited ontosubstrate 110 and the electrode then formed overlying the ion conductor.

An amount of conductive material such as metal dissolved in an ionconducting material such as chalcogenide may depend on several factorssuch as an amount of metal available for dissolution and an amount ofenergy applied during the dissolution process. However, when asufficient amount of metal and energy are available for dissolution inchalcogenide material using photodissolution, the dissolution process isthought to be self limiting, substantially halting when the metalcations have been reduced to their lowest oxidation state. In the caseof As₂S₃—Ag, this occurs at Ag₄As₂S₃=2Ag₂S+As₂S, having a silverconcentration of about 44 atomic percent. If, on the other hand, themetal is dissolved in the chalcogenide material using thermaldissolution, a higher atomic percentage of metal in the solid solutionmay be obtained, provided a sufficient amount of metal is available fordissolution.

In accordance with a further embodiment of the invention, the solidsolution is formed by photodissolution to form a macrohomogeneousternary compound and additional metal is added to the solution usingthermal diffusion (e.g., in an inert environment at a temperature ofabout 85° C. to about 150° C.) to form a solid solution containing, forexample, about 30 to about 50, and preferably about 34 atomic percentsilver. Ion conductors having a metal concentration above thephotodissolution solubility level facilitates formation ofelectrodeposits that are thermally stable at operating temperatures(typically about 85° C. to about 150° C.) of devices 100 and 200.Alternatively, the solid solution may be formed by thermally dissolvingthe metal into the ion conductor at the temperature noted above;however, solid solutions formed exclusively from photodissolution arethought to be less homogeneous than films having similar metalconcentrations formed using photodissolution and thermal dissolution.

Ion conductor 140 may also include a filler material, which fillsinterstices or voids. Suitable filler materials include non-oxidizableand non-silver based materials such as a non-conducting, immisciblesilicon oxide and/or silicon nitride, having a cross-sectional dimensionof less than about 1 nm, which do not contribute to the growth of anelectrodeposit. In this case, the filler material is present in the ionconductor at a volume percent of up to about 5 percent to reduce alikelihood that an electrodeposit will spontaneously dissolve into thesupporting ternary material as the device is exposed to elevatedtemperature, which leads to more stable device operation withoutcompromising the performance of the device. Ion conductor 140 may alsoinclude filler material to reduce an effective cross-sectional area ofthe ion conductor. In this case, the concentration of the fillermaterial, which may be the same filler material described above buthaving a cross-sectional dimension up to about 50 nm, is present in theion conductor material at a concentration of up to about 50 percent byvolume. The filler material may also include metal such as silver orcopper to fill the voids in the ion conductor material.

In accordance with one exemplary embodiment of the invention, ionconductor 140 includes a germanium-selenide glass with silver diffusedin the glass. Germanium selenide materials are typically formed fromselenium and Ge(Se)_(4/2) tetrahedra that may combine in a variety ofways. In a Se-rich region, Ge is 4-fold coordinated and Se is 2-foldcoordinated, which means that a glass composition nearGe_(0.20)Se_(0.80) will have a mean coordination number of about 2.4.Glass with this coordination number is considered by constraint countingtheory to be optimally constrained and hence very stable with respect todevitrification. The network in such a glass is known to self-organizeand become stress-free, making it easy for any additive, e.g., silver,to finely disperse and form a mixed-glass solid solution. Accordingly,in accordance with one embodiment of the invention, ion conductor 140includes a glass having a composition of Ge_(0.17)Se_(0.83) toGe_(0.25)Se_(0.75).

The composition and structure of ion conductor 140 material oftendepends on the starting or target material used to form the conductor.Generally, it is desired to form a homogenous material layer forconductor 140 to facilitate reliable and repeatable device performance.In accordance with one embodiment of the invention, a target forphysical vapor deposition of material suitable for ion conductor 140 isformed by selecting a proper ampoule, preparing the ampoule, maintainingproper temperatures during formation of the glass, slow rocking thecomposition, and quenching the composition.

Volume and wall thickness are important factors for consideration inselecting an ampoule for forming glass. The wall thickness must be thickenough to withstand gas pressures that arise during the glass formationprocess and are preferably thin enough to facilitate heat exchangeduring the formation process. In accordance with exemplary embodiment ofthe invention, quartz ampoules with a wall thickness of about 1 mm areused to form Se and Te based chalcogenide glasses, whereas quartzampoules with a wall thickness of about 1.5 mm are used to formsulfur-based chalcogenide glasses. In addition, the volume of theampoule is preferably selected such that the volume of the ampoule isabout five times greater than the liquid glass precursor material.

Once the ampoule is selected, the ampoule is prepared for glassformation, in accordance with one embodiment of the invention, bycleaning the ampoule with hydrofluoric acid, ethanol and acetone, dryingthe ampoule for at least 24 hours at about 120° C., evacuating theampoule and heating the ampoule until the ampoule turns a cherry redcolor and cooling the ampoule under vacuum, filling ampoule with chargeand evacuating the ampoule, heating the ampoule while avoiding meltingof the constituents to desorb any remaining oxygen, and sealing theampoule. This process reduces oxygen contamination, which in turnpromotes macrohomogeneous growth of the glass.

The melting temperature of the glass formation process depends on theglass material. In the case of germanium-based glasses, sufficient timefor the chalcogen to react at low temperature with all availablegermanium is desired to avoid explosion at subsequent elevatedtemperatures (the vapor pressure of Se at 920° C. is 10 ATM. and 20 ATM.for S at 720° C.). To reduce the risk of explosion, the glass formationprocess begins by ramping the ampoule temperature to about 300° C. forselenium-based glasses (about 200° C. for sulfur-based glasses) over theperiod of about an hour and maintaining this temperature for about 12hours. Next, the temperature is elevated slowly (about 0.5° C./min) upto a temperature about 50° C. higher than the liquidus temperature ofthe material and the ampoule remains at about this temperature for about12 hours. The temperature is then elevated to about 940° C. to ensuremelting of all non-reacted germanium for Se-based glasses or about 700°C. for S-based glasses. The ampoule should remain at this elevatedtemperature for about 24 hours.

The melted glass composition is preferably slow rocked at a rate ofabout 20/minute at least about six hours to increase the homogeneity ofthe glass.

Quenching is preferably performed from a temperature at which the vaporsand the liquid are in an equilibrium to produce vitrification of thedesired composition. In this case, the quenching temperature is about50° C. over the liquidus temperature of the glass material.Chalcogenide-rich glasses include a range of concentrations in whichunder-constrained and over-constrained glasses exist. In cases where theglass composition coordinated number is far from the optimalcoordination (e.g., coordination numbers of about 2.4 for Ge—Se systems)the quenching rate has to be fast enough in order to ensurevitrification, e.g., quenching in ice-water or an even stronger coolantsuch as a mixture of urea and ice-water. In the case of optimallycoordinated glasses, quenching can be performed in air at about 25° C.

In accordance with one exemplary embodiment of the invention, at least aportion of structure 100 is formed within a via of an insulatingmaterial 150. Forming a portion of structure 100 within a via of aninsulating material 150 may be desirable because, among other reasons,such formation allows relatively small structures, e.g., on the order of10 nanometers, to be formed. In addition, insulating material 150facilitates isolating various structures 100 from other electricalcomponents.

Insulating material 150 suitably includes material that preventsundesired diffusion of electrons and/or ions from structure 100. Inaccordance with one embodiment of the invention, material 150 includessilicon nitride, silicon oxynitride, polymeric materials such aspolyimide or parylene, or any combination thereof

A contact 165 may suitably be electrically coupled to one or moreelectrodes 120,130 to facilitate forming electrical contact to therespective electrode. Contact 165 may be formed of any conductivematerial and is preferably formed of a metal such as aluminum, aluminumalloys, tungsten, or copper.

In accordance with one embodiment of the invention, structure 100 isformed by forming electrode 130 on substrate 110. Electrode 130 may beformed using any suitable method such as, for example, depositing alayer of electrode 130 material, patterning the electrode material, andetching the material to form electrode 130. Insulating layer 150 may beformed by depositing insulating material onto electrode 130 andsubstrate 110 and forming vias in the insulating material usingappropriate patterning and etching processes. Ion conductor 140 andelectrode 120 may then be formed within insulating layer 150 bydepositing ion conductor 140 material and electrode 120 material withinthe via. Such ion conductor and electrode material deposition may beselective—i.e., the material is substantially deposited only within thevia, or the deposition processes may be relatively non-selective. If oneor more non-selective deposition methods are used, any excess materialremaining on a surface of insulating layer 150 may be removed using, forexample, chemical mechanical polishing and/or etching techniques.Barrier layers 155 and/or 255 may similarly be formed using any suitabledeposition and/or etch processes.

Information may be stored using programmable structures of the presentinvention by manipulating one or more electrical properties of thestructures. For example, a resistance of a structure may be changed froma “0” or off state to a “1” or on state during a suitable writeoperation. Similarly, the device may be changed from a “1” state to a“0” state during an erase operation. In addition, as discussed in moredetail below, the structure may have multiple programmable states suchthat multiple bits of information are stored in a single structure.

Write Operation

FIG. 3 illustrates current-voltage characteristics of a programmablestructure (e.g. structure 200) in accordance with the present invention.In the illustrated embodiment, via diameter, D, is about 4 microns,conductor 140 is about 35 nanometers thick and formed of Ge₃Se₇—Ag (nearAs₈Ge₃Se₇), electrode 130 is indifferent and formed of nickel, electrode120 is formed of silver, and barrier 255 is a native nickel oxide. Asillustrated in FIG. 3, current through structure 200 in an off state(curve 310) begins to rise upon application of a bias of over about onevolt; however, once a write step has been performed (i.e., anelectrodeposit has formed), the resistance through conductor 140 dropssignificantly (i.e., to about 200 ohms), illustrated by curve 320 inFIG. 3. As noted above, when electrode 130 is coupled to a more negativeend of a voltage supply, compared to electrode 120, an electrodepositbegins to form near electrode 130 and grow toward electrode 120. Aneffective threshold voltage (i.e., voltage required to cause growth ofthe electrodeposit and to break through barrier 255, thereby couplingelectrodes 320, 330 together is relatively high because of barrier 255.In particular, a voltage V≧V_(T) must be applied to structure 200sufficient to cause electrons to tunnel through barrier 255 (whenbarrier 255 comprises an insulating layer) to form the electrodepositand to overcome the barrier (e.g., by tunneling through or leakage) andconduct through conductor 140 and at least a portion of barrier 255.

In accordance with alternate embodiments of the invention, where noinsolating barrier layer is present, an initial “write” thresholdvoltage is relatively low because no insulative barrier is formedbetween, for example, ion conductor 140 and either of the electrodes120, 130.

Read Operation

A state of the device (e.g., 1 or 0) may be read, without significantlydisturbing the state, by, for example, applying a forward or reversebias of magnitude less than a voltage threshold (about 1.4 V for astructure illustrated in FIG. 3) for electrodeposition or by using acurrent limit which is less than or equal to the minimum programmingcurrent (the current which will produce the highest of the on resistancevalues). A current limited (to about 1 milliamp) read operation is shownin FIG. 3. In this case, the voltage is swept from 0 to about 2 V andthe current rises up to the set limit (from 0 to 0.2 V), indicating alow resistance (ohmic/linear cur rent-voltage) “on” state. Another wayof performing a non-disturb read operation is to apply a pulse, with arelatively short duration, which may have a voltage higher than theelectrochemical deposition threshold voltage such that no appreciableFaradaic current flows, i.e., nearly all the current goes topolarizing/charging the device and not into the electrodepositionprocess.

Erase Operation

A programmable structure (e.g., structure 200) may suitably be erased byreversing a bias applied during a write operation, wherein a magnitudeof the applied bias is equal to or greater than the threshold voltagefor electrodeposition in the reverse direction. In accordance with anexemplary embodiment of the invention, a sufficient erase voltage(V≧V_(T)) is applied to structure 200 for a period of time which dependson the strength of the initial connection but is typically less thanabout 1 millisecond to return structure 200 to its “off” state having aresistance well in excess of a million ohms. In cases where theprogrammable structure does not include a barrier between conductor 140and electrode 120, a threshold voltage for erasing the structure is muchlower than a threshold voltage for writing the structure because, unlikethe write operation, the erase operation does not require electrontunneling through a barrier or barrier breakdown.

Control of Operational Parameters

The concentration of conductive material in the ion conductor can becontrolled by applying a bias across the programmable device. Forexample, metal such as silver may be taken out of solution by applying anegative voltage in excess of the reduction potential of the conductivematerial. Conversely, conductive material may be added to the ionconductor (from one of the electrodes) by applying a bias in excess ofthe oxidation potential of the material. Thus, for example, if theconductive material concentration is above that desired for a particulardevice application, the concentration can be reduced by reverse biasingthe device to reduce the concentration of the conductive material.Similarly, metal may be added to the solution from the oxidizableelectrode by applying a sufficient forward bias. Additionally, it ispossible to remove excess metal build up at the indifferent electrode byapplying a reverse bias for an extended time or an extended bias overthat required to erase the device under normal operating conditions.Control of the conductive material may be accomplished automaticallyusing a suitable microprocessor.

This technique may also be used to form one of the electrodes frommaterial within the ion conductor material. For example, silver from theion conductor may be plated out to form the oxidizable electrode. Thisallows the oxidizable electrode to be formed after the device is fullyformed and thus mitigates problems associated with conductive materialdiffusing from the oxidizable electrode during manufacturing of thedevice.

As noted above, in accordance with yet another embodiment of theinvention, multiple bits of data may be stored within a singleprogrammable structure by controlling an amount of electrodeposit whichis formed during a write process. An amount of electrodeposit that formsduring a write process depends on a number of coulombs or chargesupplied to the structure during the write process, and may becontrolled by using a current limit power source. In this case, aresistance of a programmable structure is governed by Equation 1, whereR_(on) is the “on” state resistance, V_(T) is the threshold voltage forelectrodeposition, and I_(LIM) is the maximum current allowed to flowduring the write operation. $\begin{matrix}{R_{on} = \frac{V_{T}}{I_{LIM}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

In practice, the limitation to the amount of information stored in eachcell will depend on how stable each of the resistance states is withtime. For example, if a structure is with a programmed resistance rangeof about 3.5 kΩ and a resistance drift over a specified time for eachstate is about ±250Ω, about 7 equally sized bands of resistance (7states) could be formed, allowing 3 bits of data to be stored within asingle structure. In the limit, for near zero drift in resistance in aspecified time limit, information could be stored as a continuum ofstates, i.e., in analog form.

A portion of an integrated circuit 402, including a programmablestructure 400, configured to provide additional isolation fromelectronic components is illustrated in FIG. 4. In accordance with anexemplary embodiment of the present invention, structure 400 includeselectrodes 420 and 430, an ion conductor 440, a contact 460, and anamorphous silicon diode 470, such as a Schottky or p-n junction diode,formed between contact 460 and electrode 420. Rows and columns ofprogrammable structures 400 may be fabricated into a high densityconfiguration to provide extremely large storage densities suitable formemory circuits. In general, the maximum storage density of memorydevices is limited by the size and complexity of the column and rowdecoder circuitry. However, a programmable structure storage stack canbe suitably fabricated overlying an integrated circuit with the entiresemiconductor chip area dedicated to row/column decode, senseamplifiers, and data management circuitry (not shown) since structure400 need not use any substrate real estate. In this manner, storagedensities of many gigabits per square centimeter can be attained usingprogrammable structures of the present invention. Utilized in thismanner, the programmable structure is essentially an additive technologythat adds capability and functionality to existing semiconductorintegrated circuit technology.

FIG. 5 schematically illustrates a portion of a memory device includingstructure 400 having an isolating p-n junction 470 at an intersection ofa bit line 510 and a word line 520 of a memory circuit. FIG. 6illustrates an alternative isolation scheme employing a transistor 610interposed between an electrode and a contact of a programmablestructure located at an intersection of a bit line 610 and a word line620 of a memory device.

FIGS. 7–10 illustrate programmable devices in accordance with anotherembodiment of the invention. The devices illustrated in FIGS. 7–10 havean electrode (e.g., the cathode during a write process) with a smallercross sectional area in contact with the ion conductor compared to thedevices illustrated in FIGS. 1–2 and 4. The smaller electrode interfacearea is thought to increase the efficiency and endurance of the devicebecause an increased percentage of ions in the solid solution are ableto take part in the electrodeposit formation process. Thus any cathodeplating from ions that do not participate in the electrodeposit processis reduced.

FIGS. 7 and 8 illustrate a cross sectional and a top cut-away view of aprogrammable device 700 including an indifferent electrode 710, anoxidizable electrode 720, and an ion conductor 730 former overlying aninsulating layer 740 such as silicon oxide, silicon nitride, or thelike.

Structure 700 is formed by depositing an indifferent electrode materiallayer and an insulating layer 750 overlying insulating layer 740. A viais then formed through layer 750 and electrode material layer 710, usingan anisotropic etch process (e.g., reactive ion etching or ion milling)such that the via extends to and/or through a portion of layer 740. Thevia is then filled with ion conductor material and is suitably doped toform a solid solution as described herein. Any excess ion conductormaterial is removed from the surface of layer 750 and electrode 730 isformed, for example using a deposition and etch process. In this case,the indifferent electrode (cathode during write process) area in contactwith ion conductor 730 is the surface area of electrode 710 about theperimeter of conductor 730, rather than the area underlying the ionconductor, as illustrated in FIGS. 1–2 and 4.

FIGS. 9 and 10 illustrate a programmable device 900 having anindifferent electrode 910, an oxidizable electrode 920, an ion conductor930 and insulating layers 940 and 950 in accordance with yet anotherembodiment of the invention. Structure 900 is similar to structure 700,except that once a via is formed through layer 750, an isotropic etchprocess (e.g., chemical or plasma) is employed to form the via throughelectrode 910, such that a sloped intersection between an ion conductor930 and electrode 910 is formed.

FIGS. 11 and 12 illustrate another programmable device 1100, with areduced electrode/ion conductor interface, in accordance with thepresent invention. Structure 1100 includes electrodes 1110 and 1120 andan ion conductor 1130, formed on a surface of an insulating material1140, rather than within a via as discussed above. In this case, theprogrammable structure is formed by defining an ion conductor 1130patter on a surface of insulating material 1140 (e.g., using depositionand etch techniques) and forming electrodes 1110 and 1120, such that theelectrodes each contact a portion of the ion conductor. In the case ofthe illustrated embodiment, the electrodes are formed overlying and incontact with both a portion of the ion conductor and the insulatingmaterial. Although the thickness of the layers may be varied inaccordance with specific applications of the device, in a preferredembodiment of the invention, the thickness of the ion conductor andelectrode films is about 1 nm to about 100 nm. Sub-lithographic lateraldimensions of portions of the device may be obtained by overexposingphotoresist used to pattern the portions and/or over etching the filmlayer.

FIG. 13 illustrates a device 1300 in accordance with yet anotherembodiment of the invention. Structure 1300 is similar to the devicesillustrated in FIGS. 7 and 8, except that the cross-sectional area ofthe ion conductor that is in contact with the electrodes is reduced byfilling a portion of a via with non-ion conductor material, rather thanetching through an electrode layer.

Structure 1300 includes electrodes 1310 and 1320 and an ion conductor1330 formed within an insulating layer 1340. In this case, ion conductor1330 is formed by creating a trench within insulating layer 1340, thetrench having a diameter indicated by D2. The trench is then filledusing, for example, interference lithography techniques or conformallylining the via with insulating material and using an anisotropic etchprocess to remove some of the insulating material, leaving a via with adiameter of D3. Structure 1300 formed using this technique may have aion conductor cross sectional area as small as about 10 nm in contactwith electrodes 1310 and 1320.

FIGS. 14–17 illustrate another embodiment of the invention, where thecross sectional area of the ion conductor/electrode interface isrelatively small. Structure 1400, illustrated in FIG. 14, includeselectrodes 1410 and 1420 and an ion conductor 1430. Structure 1400 isformed in a manner similar to structure 700, except that the ionconductor material is deposited conformally, using, for example chemicalvapor deposition or physical vapor deposition, into a trench, and thetrench is not filled with the ion conductor material.

Structure 1500 is similar to structure 1400, except that an ionconductor 1530 is formed by etching a portion of ion conductor 1430,such that a via 1540 is formed through to electrode 1410. Structure 1600is similar to structure 1500 and is formed by conformally depositing theion conductor material as described above and then removing the ionconductor material from a surface of insulating material 1450 prior todepositing electrode 1420 material. Finally, structure 1700 may beformed by selectively deposing the ion conductor 1730 material into onlya portion of the trench formed in insulating material 1450 (e.g., usingangled deposition and/or shadowing techniques), removing any excess ionconductor material on the surface of insulator 1450, and forming anelectrode 1720 overlying the insulator and in contact with ion conductor1730.

FIGS. 18 and 19 illustrate yet another embodiment of the invention,where a pillar or wall within a trench is used to reduce across-sectional area of the interface between the ion conductor and oneor more electrodes. Structure 1800, illustrated in FIG. 18, includeselectrodes 1810 and 1820 and an ion conductor 1830 formed within aninsulating layer 1840. In addition, structure 1800 includes a pillar1850 of insulating material (e.g., insulating material used to formlayer 1840), formed within a trench within layer 1840. Structure 1800may be formed using the shadowed deposition technique discussed above.Structure 1900 is similar to structure 1800, except structure 1900includes a partial pillar 1950 and an ion conductor 1930, which fillsthe remaining portion of the formed trench.

FIG. 20 illustrates yet another structure 2000 in accordance with thepresent invention. Structure 2000 includes electrodes 2010 and 2020 andan ion conductor 2030 formed within an insulating layer 2040. Structure2000 is formed using an anisotropic or a combination of an anisotropicand an isotropic etch processes to form a tapered via. Ion conductor2030 is then formed within the trench using techniques previouslydescribed.

FIGS. 21–24 illustrate programmable devices in accordance with yetanother embodiment of the invention. The structures illustrated in FIGS.21–24 include a floating electrode, which allows multiple bits ofinformation to be stored within a single programmable device.

Structure 2100 includes a first electrode 2110, a second, floatingelectrode 2120, a third electrode 2130, ion conductor portions 2140 and2150, which may all be formed on a substrate or wholly or partiallyformed within a via as described above. Although structure 2100 isillustrated in a vertical configuration, the structure may be formed ina horizontal configuration, similar to structure 1100. In accordancewith one aspect of this embodiment, the first and third electrodes areformed of an indifferent electrode and the second electrode is formed ofan oxidizable electrode material. Alternatively, the first and thirdelectrodes may be formed of oxidizable electrode material and thesecond, floating electrode may be formed of an indifferent electrodematerial. In either case, the structure includes two “half cells,” whereeach half cell functions as a programmable device described above inconnection with FIG. 1. Each half cell is preferably configured suchthat the resistance of one half cell differs from the resistance of theother half cell when both cells are in an erased state.

In the case when floating electrode 2120 is formed of oxidizableelectrode material, bits of data may be stored as follows. The overallimpedance of structure 2100 is approximately equal to the resistance ofportions 2140 and 2150. When no electrodeposit is formed within eitherportion, this high resistance state may be represent by the state 00.When a voltage is applied to structure 2100, such that electrode 2130 ispositive relative to electrode 2110 and the applied bias is greater thatthe threshold voltage required to form an electrodeposit in portion2140, an electrodeposit 2160 will form through conductor portion 2140from electrode 2110 toward floating electrode 2120 as illustrated inFIG. 22. Under this condition, an electrodeposit will not form withinconductor portion 2150 because portion 2150 is under a reverse biascondition and thus will not support growth of an electrodeposit. Thegrowth of the electrodeposit will change the impedance of portion 2140from Z₁ to Z₁′, thus changing the overall impedance of structure 2100,which may be represent by the state 01. The current level used to formelectrodeposit 2160 should be selected such that it is sufficiently low,allowing the electrodeposit to be dissolved upon application of asufficient reverse bias. A third state may be formed by reversing thepolarity of the applied bias across electrodes 2110 and 2130, such thatmost of the voltage drop occurs across the high resistance ion conductorportion 2150 and formation of an electrodeposit 2170 begins, asillustrated in FIG. 23, without causing electrodeposit 2160 to dissolve.The impedance of portion 2150 changes from Z₂ to Z₂′, and the overallimpedance of structure 2100 is Z₁′ plus Z₂′, which may be represented bythe state 11. Once both half cells are in the write state,electrodeposit 2160 and/or 2170 may be dissolved by applying asufficient bias across one or both of the half cells. Electrodeposit2170 can be erased, for example, by sufficiently negatively biasingelectrode 2130 with respect to electrode 2110, which may be representedby a state 00. The four possible states, along with the current limitused to form the state, are represented in table 1 below.

TABLE 1 Current Z half- Z half- Seq # Polarity limit cell 1 cell 2State/value 1 Sub-threshold Zero Z₁ Z₂ 00 2 Upper + Lower − Low Z₁′ Z₂01 3 Upper − Lower + Low Z₁′ Z₂′ 11 4 Upper − Lower + High Z₁ Z₂′ 10

Structure 2100 can be changed to 11 from state 10 by applying a lowcurrent limit bias to grow electrodeposit 2150 in portion 2140.Similarly, structure 2100 can be changed from state 11 to state 01 bydissolving electrodeposit 2170 by applying a relatively high currentlimit bias such that upper electrode 2130 is positive with respect tolower electrode 2110. Finally, structure 2100 can be returned to state00 using a short current pulse to thermally dissolve electrodeposit2160, using a current which is high enough to cause localized heating ofthe electrodeposit. This will increase the metal concentration in thehalf-cell but this excess metal can be removed electrically from thecell by plating it back onto the floating electrode. This sequence issummarized in table 2 below.

TABLE 2 Current Z half- Z half- Seq # Polarity limit cell 1 cell 2State/value 4 Existing state — Z₁ Z₂′ 10 5 Upper + Lower − Low Z₁′ Z₂′11 6 Upper + Lower − High Z₁′ Z₂ 01 7 Upper + Lower − Thermal Z₁ Z₂ 00

Other write and erase sequences are also possible (as are otherdefinitions of the various states represented by the half-cellimpedances). For example, it is possible to go from state 00 to eitherstate 01 or state 10, depending on the write polarity chosen. Similarly,it is possible to go from state 11 to either state 10 or state 01. It isalso possible to go from state 11 to state 00 by the application of acurrent pulse (in either direction) which is high and short enough tothermally dissolve the electrodeposits in both half-cellssimultaneously.

In addition to storing information in digital form, structure 2100 canalso be used as a noise-tolerant, low energy anti-fuse element for usein field programmable gate arrays (FPGAs) and field configurablecircuits and systems. Most physical anti-fuse technologies require largecurrents and voltages to make a permanent connection. The need for suchhigh energy state-switching stimuli is generally considered to besomewhat beneficial as this reduces the likelihood of the anti-fuseaccidentally forming a connection in electrically noisy situations.However, the use of high voltages and large currents on chip represent asignificant problem as all components in the programming circuits aretypically sized accordingly and the high energy consumption reducesbattery life in portable systems.

FIGS. 25–29 illustrate structures in accordance with another embodimentof the invention in which multiple programmable devices include a commonelectrode (e.g., the devices share a common anode or cathode. Formingstructures in which multiple structures share a common electrode isadvantageous because such structures allow a higher density of cells tobe formed on a given substrate surface area.

FIGS. 25 and 26 illustrate a structure 2500, having a horizontalconfiguration and a common electrode. Structure 2500 includes anelectrical connector 2510 coupled to a common surface electrode 2520,electrodes 2530 and 2540, and ion conductor portions 2550 and 2560overlying an insulating layer 2170. Structure 2500 may be used to formword and bit lines as described above by forming a row of electrodes(e.g., anodes) coupled to conductor 2510, and columns of oppositely biaselectrodes (e.g., cathodes) running perpendicular to electrodes 2520. Aconductive plug, formed of any suitably conducting material can be usedto electrically couple electrode 2520 to conductor 2510. Althoughillustrated with a horizontal configuration, common electrode structuresin accordance with this embodiment may be formed using structures havinga vertical configuration as described herein.

FIGS. 27 and 28 illustrate additional structures 2700 and 2800 having acommon electrode shared between two or more devices Structures 2700 and2800 include a common electrode, electrodes 2720 and 2725, ionconductors 2730,2735 and 2830, 2835 respectively, and insulating layers2740 and 2750. Structures 2700 and 2800 may be formed using techniquesdescribed above in connection with FIGS. 15 and 16—e.g., by conformallydepositing ion conductor material within a trench of an insulatinglayer. In accordance with another embodiment of the invention,directional deposition may be used to form a structure similar tostructure 1700. Structures 2700 and 2800 each include two programmabledevices including common electrode 2710 an ion conductor (e.g.,conductor 2735) and another electrode (e.g., electrode 2725). Dielectricmaterial 2750 is an insulating material that does not interfere withsurface electrodeposit growth, such as silicon oxides, silicon nitrides,and the like.

FIG. 29 illustrates a structure 2900 including multiple programmabledevices 2902–2916 formed about a common electrode 2920. Each of thedevices 2902–2916 may be formed using the method described above inconnection with FIG. 21. In the embodiment illustrated in FIG. 29, eachof electrodes 2930–2936 and 2938–2944 may be coupled together in adirection perpendicular to the direction of common electrode 2920, suchthat electrode 2920 forms a bit line and electrodes 2930–2936 andelectrodes 2938–2944 form word lines. Structure 2900 may operate and beprogrammed in a manner similar to structure 2100 described above.

In accordance with other embodiments of the present invention, aprogrammable structure or device stores information by storing a chargeas opposed to growing an electrodeposit. A capacitance of a structure ordevice is altered by applying a bias across electrodes of the devicesuch that positively charged ions migrate toward one of the electrodes.If the applied bias is less that a write threshold voltage, no shortwill form between the electrodes. Capacitance of the structure changesas a result of the ion migration. When the applied bias is removed, themetal ions tend to diffuse away from the electrode or a barrierproximate the electrode. However, an interface between an ion conductorand a barrier is generally imperfect and includes defects capable oftrapping ions. Thus, at least a portion of ions remain at or proximatean interface between a barrier and an ion conductor. If a write voltageis reversed, the ions may suitably be dispersed away from the interface.

A programmable structure in accordance with the present invention may beused in many applications which would otherwise utilize traditionaltechnologies such as EEPROM, FLASH or DRAM. Advantages provided by thepresent invention over present memory techniques include, among otherthings, lower production cost and the ability to use flexiblefabrication techniques which are easily adaptable to a variety ofapplications. The programmable structures of the present invention areespecially advantageous in applications where cost is the primaryconcern, such as smart cards and electronic inventory tags. Also, anability to form the memory directly on a plastic card is a majoradvantage in these applications as this is generally not possible withother forms of semiconductor memories.

Further, in accordance with the programmable structures of the presentinvention, memory elements may be scaled to less than a few squaremicrons in size, the active portion of the device being less than onmicron. This provides a significant advantage over traditionalsemiconductor technologies in which each device and its associatedinterconnect can take up several tens of square microns.

Additionally, the devices of the present invention require relativelylow energy and do not require “refreshing.” Thus, the devices are wellsuitable for portable device applications.

Although the present invention is set forth herein in the context of theappended drawing figures, it should be appreciated that the invention isnot limited to the specific form shown. For example, while theprogrammable structure is conveniently described above in connectionwith programmable memory devices, the invention is not so limited; thestructure of the present invention may suitably be employed asprogrammable active or passive devices within a microelectronic circuit.Furthermore, although only some of the devices are illustrated asincluding buffer, barrier, or transistor components, any of thesecomponents may be added to the devices of the present invention. Variousother modifications, variations, and enhancements in the design andarrangement of the method and apparatus set forth herein, may be madewithout departing from the spirit and scope of the present invention asset forth in the appended claims.

1. A method of forming a programmable microelectronic structure, themethod comprising the steps of: providing a substrate; forming a layerof electrode material of a first type overlying the substrate; formingan insulating layer overlying the layer of electrode material of a firsttype; forming a via through the insulating layer and the layer ofelectrode material of a first type; depositing ion conductor materialinto the via; and forming an electrode of a second type overlying theion conductor material.
 2. The method of claim 1, wherein the step offorming a via includes isotropically etching the insulating layer. 3.The method of claim 1, wherein the step of forming a via includesanisotropically etching the insulating layer.
 4. The method of claim 1,wherein the step of forming a via includes isotropically etching thelayer of electrode material of a first type.
 5. The method of claim 1,wherein the step of forming a via includes anisotropically etching thelayer of electrode material of a first type.
 6. The method of claim 1,further comprising the step of applying a bias across the electrodematerial of the first type and the electrode material of the second typeto manipulate a concentration of conductive material in the ionconductor.
 7. The method of claim 1, further comprising the step ofapplying a bias across the electrode material of the first type and theelectrode material of the second type to manipulate an amount ofconductive material present in one of the electrode material of thefirst type and the electrode material of the second type.
 8. The methodof claim 1, wherein the step of depositing ion conductor materialcomprises depositing germanium onto a surface and reacting the germaniumwith H2Se.
 9. The method of claim 1, wherein the step of depositing ionconductor material comprises depositing arsenic onto a surface andreacting the arsenic with H2Se.
 10. The method of claim 1, wherein thestep of depositing ion conductor material comprises depositing germaniumonto a surface and reacting the germanium with H2S.
 11. The method ofclaim 1, wherein the step of depositing ion conductor material comprisesdepositing arsenic onto a surface and reacting the arsenic with H2S. 12.A method of forming a programmable microelectronic device, the methodcomprising the steps of: forming an ion conductor structure overlying asubstrate; depositing an electrode material layer overlying the ionconductor structure; and patterning the electrode material layer to formelectrodes in contact with the ion conductor structure wherein the stepof forming an ion conductor structure comprises depositing germaniumonto a surface and reacting the germanium with H₂Se.
 13. A method offorming an electronic device, the method comprising the steps of:forming a first electrode on a surface of a substrate; depositing afirst insulating layer over a surface of a the first electrode; forminga via in the first insulating layer; depositing a second insulatingmaterial within a portion of the via; depositing ion conductor materialwithin a portion of the via; and forming a second electrode overlyingthe ion conductor.
 14. The method of fanning an electronic device ofclaim 13, wherein the step of depositing ion conductor materialcomprises the step of deposing the ion conductor material within a viaformed in the second insulating material.
 15. The method of forming anelectronic device of claim 13, wherein the step of depositing a secondinsulating material comprises using a directional deposition technique.16. The method of fanning an electronic device of claim 13, wherein thestep of depositing an ion conductor material comprises forming aconformal layer of ion conductor material.
 17. The method of forming anelectronic device of claim 13, further comprising the step of removing aportion of the ion conductor material from a surface of the firstinsulating material.